In a general sense, this invention relates to protective coatings for metal substrates. More specifically, it pertains to methods and compositions useful for removing chromide coatings from high temperature substrates, e.g., turbine engine components.
Metal alloys are often used in industrial environments which include extreme operating conditions. As an example, gas turbine engines are often subjected to repeated thermal cycling during operation. The standard operating temperature of turbine engines continues to be increased, to achieve improved fuel efficiency. In a turbine engine, blades and vanes are arranged in stages according to the pressure and temperature of exhaust gasses which impinge upon them.
The turbine engine components (as well as other industrial parts) are often formed of superalloys, which can withstand a variety of extreme operating conditions. The superalloys are strong, creep-resistant, and fatigue-resistant. However, they are still susceptible to progressive damage by oxidation, hot corrosion, and erosion, when exposed to the hot combustion gasses which flow through the turbine. Therefore, the components are usually covered with various types of protective coatings.
Different protective coatings are used for components in the various stages of the turbine engine. Usually, thermal barrier coating (TBC) systems or aluminum-containing coatings are employed in the high-pressure stages. Chromide coatings are often favored for the low-pressure stages of the turbine, which are typically exposed in service to intermediate-range temperatures.
The protective coatings are usually applied during fabrication of the turbine components. Many techniques are available for applying the coatings, such as “pack processes” or some form of vapor deposition. U.S. Pat. Nos. 4,148,936 and 6,283,715 describe several of the deposition techniques for chromide coatings. As mentioned below, the chromium layer usually interdiffuses with the base metal in the substrate.
The protective coatings on turbine engines can degrade during service, due to continued exposure to hot exhaust gasses and temperature changes during the operating cycles of the engine. Moreover, the coatings can be damaged during handling, in the course of manufacturing, installation, and inspection. Thus, it is sometimes necessary to repair the components, particularly airfoils, and return those components to service.
During repair, the protective coatings (e.g., the chromide coatings) are often removed to allow inspection and repair of the underlying substrate. Removal is typically carried out by immersing the component in a stripping solution. In the past, common stripping solutions were based on one or more strong mineral acids (e.g., hydrochloric acid, sulfuric acid, nitric acid, and hydrofluoric acid), as well as other additives.
The prior art stripping compositions are sometimes effective for removing chromide coatings from turbine substrates. However, there are some disadvantages to their use. For example, the mineral acid compositions often emit an excessive amount of hazardous, acidic fumes. Due to environmental, health and safety concerns, such fumes must be scrubbed from ventilation exhaust systems.
Moreover, the mineral acid compositions tend to attack the substrate, pitting the base metal. The mineral acid-based processes are generally non-selective, and can result in undesirable loss of the substrate material. This material loss can lead to changes in critical dimensions, e.g., turbine airfoil wall thickness. The material loss can also lead to structural degradation of the substrate alloy, e.g., by way of intergranular attack.
Furthermore, the chromide coatings sometimes have a great deal of adherence to the substrate, and are not effectively removed with the strong mineral acids. In those instances, other techniques must be employed, such as grit blasting. However, grit blasting is a labor-intensive process that is usually carried out on a piece-by-piece basis. Special care must sometimes be taken, to prevent grit-blasting damage to the substrate or any protective coating not being removed during the turbine component overhaul.
In view of some of the drawbacks of the prior art, new processes for removing chromide coatings from metal substrates would be welcome in the art. The processes should be capable of removing substantially all of the coating material, while not attacking the substrate itself. It would also be desirable if the processes did not result in the formation of an unacceptable amount of hazardous fumes.
In some instances, the processes should also be compatible with processes being used to remove other coatings from the component, e.g., metallic overlay coatings. The processes should also exhibit some degree of selectivity. For example, they should effectively remove the chromide coating while substantially preserving the substrate.